Content addressable memories are known which comprise a plurality of RAM cells connected in an array of rows and columns. In addition to conventional read and write operations to cells in the memory array, a content addressable memory (CAM) permits an association operation in which a data word is input to the CAM and the contents of each row of RAM cells are tested to see if they hold the same data word. Each row of cells has a match line and if a match is found between the data word which is input and the contents of a row of cells then a signal is provided on the appropriate match line indicating the row where a match has been found. The data word which is input is normally applied to each row simultaneously and to all cells in each row simultaneously. Commonly a match line for each row which is to be subject to an association operation is precharged and the RAM cells in each row are arranged to discharge the match line if no match is found with the corresponding part of the input word. In this way the time taken to determine whether a match is found during an association operation depends on the time taken to discharge each match line where no match is found. Each row of cells may have a plurality of cells such as for example 32 bits where data is held in 32 bit words. A mismatch may arise from failure to match on a single cell in the 32 bit word. Consequently the time allocated to determine whether or not a match arises must be sufficient to handle the worst possible case where only one cell fails to provide a match in that particular row of cells.
It is an object of the present invention to provide an improved content addressable memory and system for accessing such a CAM where the speed of operation is improved.